1. FIELD OF THE INVENTION
The present invention relates to a vibration detecting apparatus, and more particularly to a vibration detecting apparatus suitable for use in an observation of vibration generated on an internal combustion engine, which is adapted to detect a generation of knocking through the observation of vibration transmitted to the outside of a cylinder of the internal combustion engine due to an interior pressure change in the cylinder and control an ignition timing so that the knocking is suppressed below a predetermined magnitude.
2. DESCRIPTION OF THE PRIOR ART
It is generally known that the ignition timing and the interior pressures in cylinders are closely correlated with each other. As the ignition timing advances, a vibration component of high frequency (generally, 6 to 9 KHz, 11 to 13 KHz, and 16 to 19 KHz), which is determined by a bore diameter of the engine cylinder and the speed of sound in combustion, increases to cause a knocking as a vibration or sound transmitted to the outside of the cylinder and a magnitude of the knocking also increases. The knocking is generally referred to as "trace knock" in early stage of its generation, "light knock" in its gradually increasing stage, and "heavy knock" in its further increasing stage. To better the efficiency of an internal combustion engine without exerting any adverse effect on the engine, it is necessary to feedback control the ignition timing so that the engine is constantly maintained in the state of trace knock. In consequence, there is a need for a vibration detecting apparatus which can operate accurately and stably under severe environmental conditions.
A conventional detecting apparatus of this kind, for example, employs an ordinary acceleration detecting apparatus for this purpose. Such conventional detecting apparatus includes a detecting apparatus which has a resonance frequency thereof set higher than a frequency band in which the knocking is generated and has a substantially flat frequency characteristic below the knocking frequency band (hereinafter referred to as "non-resonance type detecting apparatus" in this specification and the claims appended thereto), and a detecting apparatus which has a resonance frequency thereof set correspondingly to the knocking frequency so that the sensitivity near the resonance frequency is much higher than that at other frequencies (referred to as "resonance type detecting apparatus", hereinafter). The resonance type detecting apparatus has a high resonance peak or quality factor Q. Accordingly, when a resonance frequency of the resonance type detecting apparatus is set to correspond to the knocking frequency, the SN ratio with respect to the knocking frequency becomes higher. In this type of detecting apparatus, however, it is inconveniently difficult to make the resonance frequency correspond to the knocking frequency and if the resonance frequency is somewhat offset from the knocking frequency, the SN ratio decreases considerably. In addition, the knocking frequency generally differs depending on the type of internal combustion engine, and even in the same internal combustion engine there are variations of knocking frequency between cylinders thereof. Moreover, the knocking frequency may fluctuate in accordance with the engine operation conditions, such as the engine speed.
To obviate the above-mentioned disadvantages, a non-resonance type detecting apparatus is taken into consideration since it is not necessary to select different detecting apparatuses according to the type of internal combustion engine. The most effective non-resonance type detecting apparatus employs as its detecting means a piezoelectric element which allows the detecting apparatus to be reduced in size, and has a resonance frequency thereof set at a high frequency where it is not affected by the knocking frequency. This detecting apparatus is mounted to an engine block, for example, of an internal combustion engine which is to be subjected to the detection of knocking. A typical conventional non-resonance type detecting apparatus of this kind is composed of a detecting means for detecting a vibration generated on an engine block of an internal combustion engine; a connector; a signal transmitter for transmitting a signal from the detecting means to the connector; and a housing for covering the detecting means and the signal transmitter to protect both of them.
The detecting means includes a piezoelectric element for converting a mechanical vibration generated on the engine block into an electric signal. The piezoelectric element is screw-mounted to one end wall portion of the housing by means of a screw, a washer and the like and biased.
Further, the connector having an output pin is mounted to the housing. The piezoelectric element and the output pin are electrically connected through a lead wire soldered to both of them.
This detecting apparatus necessitates a soldering work in order to connect the piezoelectric element with the output pin through the lead wire and therefore it is troublesome to assemble. In addition, the detecting apparatus is insufficient in reliability since there is a fear that in the case of a use over a long period of time, a vibration may cause the lead wire to be disconnected or separated at its soldered portions. Moreover, since the piezoelectric element is screw-mounted by means of a screw and the like, the piezoelectric element requires a bore for receiving the screw. In consequence, there is a need for a process for boring after calcination and the detecting area decreases, so that the output impedance is increased and then the detecting apparatus is easily affected by noises.
In another conventional non-resonance type detecting apparatus, an initially conical spring made of an electrically conductive material is disposed between a piezoelectric element within a housing and a connector by which the housing is capped. Although this detecting apparatus has a simple construction and is readily assembled, the reliability thereof is insufficient in the case of a use over a long period of time, since the initially conical spring, which biases the piezoelectric element, simultaneously urges the connector so as to be drawn out from the housing.